1. Field of the Invention
This invention relates to a method of adjusting read-out processing conditions, i.e. read-out conditions for final read-out and/or image processing conditions, for a radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays .alpha.-rays, 62 -rays, .gamma.-rays cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon. Then, final read-out is carried out by exposing the stimulable phosphor sheet carrying the radiation image stored thereon to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light to obtain electric image signals for use in reproduction of a visible image. An image processing is carried out on the final read-out image signals obtained by the final read-out, and the radiation image of the object is reproduced as a visible image by use of the processed image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, the range of the level of the radiation energy stored on the stimulable phosphor sheet is caused to fluctuate among radiation images by changes in the object, the image recording portion thereof, radiation dose, or the like.
However, in the aforesaid radiation image recording and reproducing system, the recorded image information, particularly the range of the level of the radiation energy or the like, of each radiation image stored on the stimulable phosphor sheet may be ascertained in advance, and the final read-out may be carried out by use of read-out conditions such as a read-out gain and a scale factor adjusted to appropriate values in accordance with the recorded image information of each radiation image. In this case, for each radiation image, it becomes possible to obtain a visible image free from adverse effects of the fluctuation in the range of the level of the radiation energy stored on the stimulable phosphor sheet and suitable for viewing, particularly for diagnostic purposes, for example, a visible image wherein the necessary object image information is always expressed within the correct density range suitable for viewing, particularly for diagnostic purposes.
Also, in the aforesaid radiation image recording and reproducing system, the image processing of the final read-out image signals obtained by the final read-out is carried out by use of image processing conditions such as gradation processing conditions adjusted for each radiation image based on the image recording portion of the object such as the head, the chest or the abdomen, and/or the image recording method such as plain image recording or contrasted image recording so that a visible image suitable for viewing, particularly for diagnostic purposes, can be obtained. However, for example, in the case where the final read-out is carried out without using the read-out conditions adjusted to appropriate values in accordance with the recorded image information of each radiation image, the image processing conditions should preferably be adjusted by considering the recorded image information of each radiation image, which has been ascertained in advance, besides the image recording portion of the object and/or the image recording method. In this manner, it becomes possible to obtain a visible image suitable for viewing, particularly for diagnostic purposes, wherein the necessary object image information is expressed within the correct density range.
Ascertaining of the image information recorded on the stimulable phosphor sheet prior to the final read-out and the image processing may be carried out by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, preliminary read-out for ascertaining the recorded image information of a radiation image stored on the stimulable phosphor sheet is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in the final read-out for detecting the image signals for use in reproduction of a visible image for viewing, particularly for diagnostic purposes, and thereafter the final read-out is carried out. The read-out conditions for the final read-out and/or the image processing conditions are adjusted based on the recorded image information ascertained by the preliminary read-out, i.e. the preliminary read-out image signals obtained by the preliminary read-out.
In the case where only the image processing conditions are to be adjusted based on the recorded image information, it is only necessary that the recorded image information be ascertained prior to the image processing. In this case, since the final read-out image signals representing the recorded image information have already been detected, the image processing conditions can be adjusted based on the final read-out image signals, and the preliminary read-out as mentioned above need not necessarily be carried out.
On the other hand, as one of the methods for adjusting the read-out conditions for the final read-out and/or the image processing conditions on the basis of the aforesaid preliminary read-out image signals and/or the final read-out image signals used in the course of adjustment of the image processing conditions (both the preliminary read-out image signals and the final read-out image signals used in the course of adjustment of the image processing conditions will hereinbelow be generically referred to as the image signals for condition adjustment), it is considered to employ a method wherein the maximum image signal level Smax and the minimum image signal level Smin of a desired image information range in the image which the image signals for condition adjustment represent are calculated from the image signals for condition adjustment, and the read-out conditions for the final read-out and/or the image processing conditions are adjusted on the basis of Smax and Smin.
As the method of adjusting the read-out conditions for the final read-out on the basis of the image signals for condition adjustment, a novel method has been proposed in, for example, Japanese Unexamined Patent Publication No. 60(1985)-156055. The proposed method comprises the steps of: determining a histogram of the preliminary read-out image signals (image signal levels), calculating the maximum image signal level Smax and the minimum image signal level Smin of a desired image information range in the histogram, and adjusting the read-out conditions for the final read-out so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in an image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
Also, as the method of adjusting the image processing conditions, for example, gradation processing conditions, on the basis of the image signals for condition adjustment, the method as mentioned above may be used by way of example. Specifically, there may be used a method comprising the steps of: determining a histogram of the image signals for condition adjustment (image signal levels), calculating the maximum image signal level Smax and the minimum image signal level Smin of a desired image information range in the histogram, and adjusting the gradation processing conditions so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Rmax and the minimum signal level Rmin of a desired input signal range in an image reproduction means (visible image output means) which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
The term "read-out conditions" as used herein means various conditions affecting the relationship between the input to the read-out means and the output thereof, for example, the relationship between the input to the photoelectric read-out means (i.e. the amount of the light emitted by the stimulable phosphor sheet) and the output thereof (i.e. the level of the electric image signal) in the aforesaid case. For example, the term "read-out conditions" means the read-out gain (sensitivity) determining the relationship between the input and the output of the read-out means, the scale factor (latitude), and the power of the stimulating rays used for read-out.
The term "image processing conditions" as used herein means various conditions affecting the relationship between the input and output of an image processing means, for example, gradation processing conditions, frequency response processing conditions, or the like.
As mentioned above, the level of the stimulating rays used in the preliminary read-out is adjusted to be lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out is adjusted to be lower than the effective energy of the stimulating rays used in the final read-out.
However, it may occur that the image information which the preliminary read-out image signals and the final read-out image signals used as, by way of example, the aforesaid image signals for condition adjustment represent contains unnecessary image information besides the desired image information, and the maximum image signal level Smax and the minimum image signal level Smin of a desired image information range cannot be calculated accurately because of the presence of the unnecessary image information. In the case where Smax and Smin are incorrect, the read-out conditions for the final read-out and/or the image processing conditions adjusted based on Smax and Smin become incorrect, and consequently it is not always possible to obtain a visible reproduced image suitable for viewing, particularly for diagnostic purposes.
By way of example, the aforesaid problems arise in the case where image recording is carried out by limitation of the irradiation field. Specifically, in the case where the aforesaid radiation image recording and reproducing system is used for medical diagnosis, portions of the human body not related to diagnosis should not be exposed to radiation since the radiation is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in the aforesaid radiation image recording and reproducing system, the irradiation field is often limited in the course of recording of a radiation image. However, in the case where image recording is carried out by limitation of the irradiation field as mentioned above, radiation scattered by the object within the irradiation field normally passes outside of the irradiation field. The scattered radiation is absorbed and stored on the stimulable phosphor sheet which exhibits high sensitivity, and therefore the histogram of the image signals (image signal levels) obtained by the preliminary read-out or the final read-out includes the image signal level caused by the scattered radiation. Since the image signal level caused by the scattered radiation outside of the irradiation field on the stimulable phosphor sheet is often higher than the image signal level inside of the irradiation field, it is not always possible to discriminate between the image signal levels inside and outside of the irradiation field in the histogram obtained by the preliminary read-out. Therefore, in the case where Smax and Smin are calculated from the histogram as mentioned above and the read-out conditions for the final read-out and/or the image processing conditions are adjusted on the basis of Smax and Smin, the minimum image signal level inside of the irradiation field is not detected as Smin, and that caused by the scattered radiation outside of the irradiation field is detected as Smin. In general, the minimum image signal level outside of the irradiation field is lower than that inside of the irradiation field. Accordingly, when the minimum image signal level outside of the irradiation field is detected as Smin, signals caused by the scattered radiation not related to diagnosis are taken within a low density range in the final read-out and/or image processing, and the density of the image of the portion related to diagnosis becomes too high. As a result, the image contrast decreases and it becomes difficult to make an accurate diagnosis.
Namely, in the case where a radiation image is recorded by limiting the irradiation field, radiation scattered by the object passes outside of the irradiation field on the stimulable phosphor sheet, and noise caused by the scattered radiation (noise representing the unnecessary image information) is contained in the image signals obtained by the preliminary read-out and the final read-out. Therefore, when the read-out conditions for the final readout and/or the image processing conditions are adjusted based on such image signals, it is not always possible to adjust the read-out conditions for the final read-out and/or the image processing conditions to appropriate values and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.
Therefore, in the case where the read-out conditions for the final read-out and/or the image processing conditions are to be adjusted based on the preliminary read-out image signals and/or the final read-out image. signals by use of the aforesaid method, the maximum image signal level Smax and the minimum image signal level Smin of a desired image information range should be calculated accurately by eliminating the adverse effects of the unnecessary image information such as the image information outside of the irradiation field, and the readout conditions for the final read-out and/or the image processing conditions should be adjusted on the basis of Smax and Smin thus calculated.
The aforesaid problems arise regardless of the kind of the recording medium on which the radiation image is to be recorded, i.e. regardless of whether the recording medium is the stimulable phosphor sheet or not.